Late Jurassic Magmatism and Stratigraphy in the Eastern Sakarya Zone, Turkey: Evidence for the Slab Breakoff of Paleotethyan Oceanic Lithosphere

• Main Authors: Dokuz, Abdurrahman, Aydincakir, Emre, Kandemir, Raif, Karsli, Orhan, Siebel, Wolfgang, Derman, Ahmet Sami, Turan, Mehmet
• Format: Article in Journal/Newspaper
• Language: English
• Published: Univ Chicago Press 2017
• Subjects: North China Craton; Kastamonu Granitoid Belt; Yusufeli-Artvin Area; Pontides Ne Turkey; Volcanic-Rocks; Calc-Alkaline; Tectonic Evolution; Gumushane Area; Trace-Element; Geodynamic Evolution; Dy; sami; Ocean Island
• Online Access: https://hdl.handle.net/20.500.12440/3096; https://doi.org/10.1086/689552

Middle-Late Jurassic Cimmerian events in Turkey have been actively discussed in the past three decades, but proposed tectonic models associated with magmatism, metamorphism, and stratigraphic features remain controversial. To address this issue, Upper Jurassic mafic lavas are investigated at three locations (Alucra, Gumushane, and Olur) in the eastern Sakarya Zone, northeastern Turkey. These lavas are submarine and form planar flows parallel with the bedding plane in the Upper Jurassic carbonate sequence near the base or just below in the clastic sedimentary rocks. The basaltic lavas show calc-alkaline features and possess Nb-Ta values and Nb/U, Nb/La, and Ce/Pb ratios that are greater than those of island arc basalts. Multielement patterns are almost hump shaped, similar to ocean island basalts, which experience Pb depletion and weak negative Nb-Ta, Zr-Hf, and Ti anomalies. The low initial (Sr-87/Sr-86) ratios (0.70372-0.70554), positive initial epsilon(Nd) values (+2.7 to +4.4), and initial Pb isotope ratios that plot between mid-ocean-ridge and ocean island basalts are consistent with a melt derived from subcontinental lithospheric mantle, metasomatized by earlier fluids from subducted sediments and plume materials from the asthenosphere. Moderate Dy/Yb ratios with an average value of 1.8 imply partial melting in the spinel-garnet transition zone at depths of similar to 70-100 km. Slab breakoff is suggested as a geodynamic mechanism that accounts for these geochemical signatures. This inference is also favored by stratigraphic and sedimentologic evidence from the Upper Jurassic-Lower Cretaceous sedimentary rocks, which is consistent with short-lived vertical (epirogenic) movements in the region. Lower-Middle Jurassic sequences are transgressive, suggesting that subduction-related extension opened a backarc basin (Neotethys) in the south of the Sakarya Zone. Upper Jurassic-Lower Cretaceous carbonates point tectonically to tranquility during carbonate deposition in the Neotethys Ocean, which seems to have been achieved by complete closure of the Paleotethys in the north. About 15-20 m.yr. later (Kimmeridgian), after first carbonate deposition, intraplate-typemafic lavas ascended up to the shelf surface of the Neotethys. This was followed by formation of disconformity surfaces and then accumulation of coarse clastic sediments. All this points to a short-lived epirogenic movement that we ascribe to the breakoff of the southward-subducting Paleotethyan oceanic lithosphere in the Late Jurassic. Research Foundation of Karadeniz Technical University [2007.118.002.2, 2009.112-005-2] This research was supported by the Research Foundation of Karadeniz Technical University (2007.118.002.2and2009.112-005-2). E. Reitter is thanked for his help during the Sr-Nd-Pb isotope analyses. We wish to thank the reviewer, D. van Hinsbergen, and Editor D. Rowley, who provided many comments that improved the manuscript. We also wish to remember the sixth author of this essay, Ahmet Sami Derman, who died on May 18, 2012. He enabled us to become aware of the mafic rocks in this study during the fieldwork for his project on the Upper Jurassic-Lower Cretaceous carbonates of the Eastern Pontides. WOS:000392384000001 2-s2.0-85009904478